Only show parse errors and command to invoke usage.

Rather than showing the usage when an error is encounted during options
parsing, show a message that describes how to invoke help instead.  This
is useful because the help is long enough now that the error is often
overlooked since it scrolls out of view.

ok @jrick
IMPORT PATH MODIFICATION CHANGESET
----------------------------------

Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.